1. Field of the Invention
The present invention relates to a process for producing an alkylene glycol from an alkylene oxide. More particularly, it relates to a process for producing an alkylene glycol with especially high efficiency.
2. Discussion of Background
An alkylene glycol, particularly ethylene glycol, is used, for example, as a raw material for synthetic fibers or resins, or as an anti-freezing liquid, and is an industrially important compound.
As a way for producing an alkylene glycol, a process of hydrolyzing an alkylene carbonate, is well known. Such a reaction is usually carried out in the presence of a catalyst for hydrolysis, and it has been proposed to use a catalyst for hydrolysis such as an alkali metal carbonate (U.S. Pat. No. 4,117,250), a molybdenum compound (JP-B-55-154927) or a tungsten compound (JP-B-55-154928) in order to increase the reaction rate.
By the use of these catalysts for hydrolysis, the hydrolysis can be accelerated, but the degree of acceleration has not been adequate. If the reaction is carried out at a higher temperature to accomplish an industrially satisfactory reaction rate, there has been a problem that the quality of the product tends to deteriorate. On the other hand, if the reaction is carried out at a lower temperature to secure the quality of the product, the reaction rate will be low, and an excessive capacity of the reactor is required to attain the predetermined productivity, or an unreacted alkylene carbonate tends to remain in the product.
Whereas, if ethylene carbonate remains after the hydrolysis in the course of production of ethylene glycol which is industrially most important, it forms an azeotropic mixture together with ethylene glycol, whereby their separation or purification tends to be difficult.
Further, in the hydrolysis of an alkylene carbonate, it is common to employ a molar ratio of water to an alkylene carbonate in the charged starting materials within a range of from about 1.3:1 to about 5.0:1. If the molar ratio is less than this range, there will be a problem that as the reaction proceeds, water will be consumed, and the water concentration will decrease, whereby the reaction rate decreases, so that it takes time to complete the reaction, and the amount of impurities formed, tends to increase. On the other hand, if water is charged in a large amount beyond this range, water will be present in the system in an amount substantially exceeding the amount consumed for the reaction, whereby there will be a problem that a large quantity of heat will be required for heating the reaction solution and separating water in the purification system.
Further, the alkylene carbonate to be used as the starting material, can be obtained by reacting an alkylene oxide with carbon dioxide gas in the presence of a carbonating catalyst. However, in a case where this step and the hydrolyzing step are carried out continuously, if the carbonating catalyst is used by recycling, the carbonating catalyst activities will gradually decrease. Accordingly, it is desired to develop a process whereby the catalyst activities will not decrease.